Fibre optical communications uses light confined in an optical fibre for transferring information over long distances. For high speed transmission over long distances, an important light source for fibre optical communication is the Distributed Feed-Back laser (DFB-laser).
Two major types of DFB-lasers exists: Buried Heterostructure (BH) and Ridge. These two types are briefly described in connection with FIG. 1a and 1b. Both have their advantages, e.g. the BH in general gives better performance and the Ridge is simpler to manufacture.
Even better performance may be obtained by adding a modulator to the DFB laser, e.g. an integrated Electro Absorption modulator (DFB-EA), since it introduces less chirp than direct modulation of the laser.
The DFB-EA component is made of a laser (DFB) and a modulator (EA). The device may be manufactured in many different ways and a popular way is to first epitaxially grow the laser material, then etch away all material not needed for the laser part and regrow new material around the laser (Butt Joint) to use for the modulator. Then a Ridge DFB-EA could be made if a contact layer is grown on top of the laser and the modulator material followed by etching the ridge. When manufacturing a Ridge DFB-EA laser, relatively high threshold currents, poor thermal properties and high contact resistance are major issues.
These issues may be taken care of by manufacturing a BH DFB laser instead, this however requires one or two extra epitaxial process steps and hence increases the complexity (and price) of the device.
In DE 3810767 A1, a photonic device is described comprising a Ridge DFB laser having a BH structure. A cladding layer is added on top of the ridge structure and a contact layer is arranged on top of that. The width of the contact layer is limited and an insulating material , such as SiO2, is arranged beside the contact layer on both sides. A metal contact is thereafter provided on top of the contact layer and the insulating material.
In an article by N. Bouadma and J. Semo with the title “1.3-μm GaInAsP/InP Buried-Ridge-Structure Laser and its Monolithic Integration with Photodetector Using Reactive Ion Beam Etching” a photonic device is shown (see FIG. 4) where isolation of the component is achieved by implantation of protons into adjacent regions. This is a difficult technique to use when the mesa structure is high.
Insulating materials, such as BCB (Benzocyclobutene), has been used in micro chip fabrication for a long time, e.g. see article by R. A. Kirchhoff, C. J. Carriere, K. J. Bruza, N. G. Rondan, and R. L. Sammler with the title “Benzocyclobutenes: A new class of high performance polymers” Science-Chemistry, Vol A28, Nos. 11 & 12, 1991, pp. 1079-1113. The material has been used in a variety of electronics applications ranging from conductive, metal-filled adhesives to high planarizing and insulating layers on silicon wafers.